1. Field of the Invention
The present invention relates to lead free glass frit, powder for manufacturing a silicon solar cell, its producing method, a metal paste composition comprising the same and a silicon solar cell, and in particular, to lead free glass frit powder for manufacturing a silicon solar cell, which is environmental friendly and improves the performance of a solar cell, its producing method, a metal paste composition comprising the same and a silicon solar cell.
2. Description of the Related Art
Recently, it is expected that conventional energy sources such as oil or charcoal will be exhausted, and thus interests in alternative energy source are increasing. Among alternative energy source, a solar cell has abundant energy sources and does not cause environmental pollution, and thus it becomes the object of attention.
The solar cell is classified into a solar heat cell that produces vapor required to run a turbine using solar heat, and a solar light cell that converts photons into electrical energy using properties of a semiconductor. Generally, the solar light cell is represented as a solar cell.
The solar cell largely includes a silicon solar cell, a compound semiconductor solar cell and a tandem solar cell according to raw material. Among them, the silicon solar cell leads the solar cell market.
FIG. 1 is a schematic cross-sectional, view illustrating a basic structure of a silicon solar cell. Referring to FIG. 1, the silicon solar cell includes a substrate 101 of a p-type silicon semiconductor, and an emitter layer 102 of an n-type silicon semiconductor. A p-n junction is formed at an interface between the substrate 101 and the emitter layer 102 in the similar way to a diode.
When light falls on a solar cell of the above-mentioned structure, electrons and electron holes create in a silicon semiconductor doped with an impurity by the photovoltaic effect. Specifically, electrons create in the emitter layer 102 of an n-type silicon semiconductor as a plurality of carriers, and electron holes create in the substrate 101 of a p-type silicon semiconductor as a plurality of carriers. The electrons and electron holes created by the photovoltaic effect are drawn toward the n-type silicon semiconductor and p-type silicon semiconductor, and move to a front electrode 103 on the emitter layer 102 and a rear electrode 104 below the substrate 101, respectively. When the front electrode 103 and the rear electrode 104 are connected to each other, electrical current flows.
A front electrode of a silicon solar cell is formed through an interface reaction between a metal paste for forming a front electrode and an anti-reflection film. Specifically, while silver contained in a metal paste becomes liquid at high temperature and then is recrystallized into a solid again, a front electrode is contacted with an emitter layer due to a punch-through phenomenon that the front electrode penetrates through an anti-reflection film through the medium of glass frit powder. The definite mechanism is disclosed in J. Hoomstra, et al., 31st IEEE PVSC Florida 2005.
The glass frit powder carries out an interface reaction with an anti-reflection film to etch the anti-reflection film. This is an oxidation-reduction reaction that a portion of elements is reduced and generated as a by-product. Conventionally, because glass frit powder contains lead oxide (PbO) as a main component, lead is reduced after an interface reaction and it causes environmental problems.
To solve this drawback, lead free glass frit powder containing bismuth oxide (Bi2O3) instead of lead oxide has been introduced. However, the bismuth oxide-based glass frit powder has lower contact resistance between an electrode and a substrate than conventional glass frit powder containing lead oxide.
Therefore, there is an urgent need for glass frit powder that is more environmentally friendly and can be used to manufacture solar cells of better performance than the conventional glass frit powder.